RMI 12330 A [N-(cis-2-phenyl-cyclopentyl)-272-cyclotridecan-2-imine hydrochloride], a compound of the lactamamide series, is a potent inhibitor of vasoactive intestinal polypeptide(VIP)- and prostaglandin(PG)-stimulated colonic secretion in the rat. This substance was tested on the adenylate cyclase system in human colonic mucosa. RMI 12330 A inhibited PGE2-, 16,16-dimethyl-PGE2- as well as VIP-sensitive adenylate cyclases in a dose-related manner. Half-maximal inhibition of hormone-stimulated enzyme activities occurred at a lactamimide concentration of about 0.15 mM. Lactamimide inhibition was non-competitive. RMI 12330 A may act as an inhibitor of colonic secretion via inhibition of hormone-sensitive adenylate cyclase. Since basal, NaF-and guanylyl-imidodiphosphate-stimulated enzyme activities were also affected by this compound, RMI 12330 A may be a non-specific inhibitor of the human colonic adenylate cyclase system.